Even perpetual calendars capable of keeping time over decades will need manual adjustment tonight as a leap second is added to 2016.
A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC) in order to keep its time of day close to the mean solar time, or UT1.
Without such a correction, time reckoned by Earth’s rotation drifts away from atomic time because of irregularities in the Earth’s rate of rotation.
Since this system of correction was implemented in 1972, 26 leap seconds have been inserted, the most recent on June 30, 2015 at 23:59:60 UTC, and the next leap second will be inserted on December 31, 2016, at 23:59:60 UTC.
